Duplicated Calendars #1

Cause

Solution

In iTunes, stop syncing your From my Mac calendars in the iTunes settings when your device is plugged in

Duplicated Calendars #2

Cause

iCloud incorrectly duplicated all your calendars: under the 'iCloud' header, all your calendars show up twice.

Solution

Uncheck the duplicated calendars

Delete the duplicated calendars on the iCloud website

Duplicated Events

Cause

iCloud incorrectly duplicated all events in the same calendar as real duplicates. You can see this if unchecking (hiding) one calendar removes all duplicates. The duplicates also show up in the native iOS calendar as well.